1. Field of the Invention
The present invention relates to a rotating electric machine for use onboard a motor vehicle.
2. Description of the Background Art
Batteries equipped onboard a motor vehicle typically supply a terminal voltage of 14 V or less. During power running of the vehicle performed with the assistance of an automotive rotating electric machine (i.e., an automotive motor), such as at engine restart from idle stop or during engine-assist operation of the vehicle, this type of battery is not able to provide a sufficient output torque due to low output voltage. Particularly at engine restart from idle stop, in which the engine once shut off at a vehicle stop is restarted by running the vehicle by only the onboard motor which works as a starter motor, the motor should preferably produce a greater starting torque. A greater motor torque is also needed for performing the engine-assist operation to give extra power in high-speed ranges. Generally, it is necessary for a low-voltage driving system to flow a larger electric current for producing greater output power. However, the amount of electric current that can be flowed through the driving system is more or less limited due to heat generation and reliability of components used in an inverter.
Additionally, in an electric driving system, a voltage drop equivalent to the product of electric current and resistance causes an energy loss in the form of Joule heat, and a torque-generating voltage expressed by the product of effective magnetic flux and motor speed decreases by an amount corresponding to this voltage drop. This means that if the terminal-to-terminal resistance of each armature coil of the automotive motor increases, the effective magnetic flux decreases by a corresponding amount, making it necessary to perform field-weakening control. A low-voltage driving system is already in a voltage saturation range (field-weakening range) from a certain point in a low-speed range and, thus, a sufficient torque is not obtained in a high-speed range in which the motor speed is relatively high (typically 300 rpm or above). Here, the voltage saturation range is a range in which the motor is controlled by field-weakening control operation by flowing a negative d-axis current Id as a voltage induced by a combination of field flux and armature flux exceeds a terminal voltage in this range. Although it is possible to increase the starting torque produced by the motor by increasing the number of turns of armature coils, electric resistance of each armature coil increases with an increase in the number of turns thereof, resulting in a reduction in the torque obtained in the high-speed range (hereinafter referred to as the high-speed torque). Conversely, if the resistance of each armature coil is reduced by decreasing the number of turns thereof, a desired level of starting torque would not be obtained.
As the conventional automotive motor is thus configured, it is difficult to obtain both a starting torque characteristic required at engine restart and a high-speed torque characteristic required in the voltage saturation range.